A fluid sensor

Abstract: A method of determining the volumetric flow rate of a fluid, which determines the volumetric flow rate of said fluid essentially independently of the properties of the fluid, comprises the following steps: guiding the volumetric fluid flow to be measured through an essentially linear flow resistor (110); generating an alternating fluid flow and superimposing the same on the volumetric fluid flow to be measured; detecting the pressure drop across the flow resistor (110); determining the steady component and the alternating component of the pressure drop detected; determining a quantity (FV) representative of a fluid property of the fluid on the basis of the alternating component which has been determined; and calculating the volumetric flow rate on the basis of the steady component which has been determined as well as on the basis of the quantity (FV) representative of the fluid property.

Description: FIELD OF THE INVENTION The present invention refers to a fluid sensor in accordance with the preamble of claim 1. DESCRIPTION OF THE PRIOR ART The present invention especially refers to sensors which operate according to the pressure measuring method and which depend on the properties of the fluid to be measured, such as the viscosity thereof, only to a minor extent. Fluid sensors for detecting a volumetric flow rate have been used for a considerable time in the field of chromatography. Various pressure measuring methods for measuring a volumetric flow rate are known. One method uses the impact pressure on a lug for the purpose of measuring the volumetric flow rate. A lug projecting into the fluid flow is here deflected by the impact pressure of the fluid flowing past the lug. This deflection is a measure of the flow in question. This method is, however, fluid-dependent, its dynamic range is limited and it has a lower measuring limit which is very high. Furthermore, the measurement depends on the position of the system and it is sensitive to concussions. Another known pressure measuring method uses the impact pressure on a shutter or nozzle for determining the volumetric flow rate. The impact pressure at the shutter depends on the density of the fluid within the measurement range. Also this known method is fluid-dependent, it has a low dynamic range and it is only suitable for high flow rates. Still another known pressure measuring method is the differential pressure measurement across an essentially linear flow resistor. This pressure measuring method makes use of a fluid sensor of the type shown in Fig. 6. The fluid sensor 600 shown in Fig. 6 comprises an essentially linear flow resistor 610, which is arranged in the fluid flow. A differential pressure sensor 620 is connected in parallel to said flow resistor 610, said differential pressure sensor being used for measuring the pressure difference across the flow resistor 610. The volumetric flow rate is then calculated on the basis of this measured pressure difference. This is, however, only possible if the properties of the fluid are known. A disadvantage of said method becomes evident in this respect, said disadvantage being that this measurement, too, is fluid-dependent. A fluid sensor in accordance with the preamble of claim 1 is known from DE-B-25 30 474. This known fluid sensor is used for measuring the breathing gas flow of a person independently of the gas type. The known sensor uses an alternating pressure pump, comprising a movable piston, for superimposing an alternating gas flow on the gas flow to be measured, and a differential pressure sensor. From WO-A-85 02463 , a device for measuring the viscosity of a fluid is known, which comprises a measuring chamber that can be filled with a standard fluid and is provided with a pressure and flow generating plunger and a pressure sensor. A measuring tube is connected through a flexible diaphragm with the measuring chamber for receiving the fluid to be measured, and a coupling chamber is provided which can be filled with a coupling fluid and which is interposed between the measuring tube and the diaphragm. SUMMARY OF THE INVENTION Taking this prior art as a basis, it is the object of the present invention to provide a fluid sensor which depends on the fluid to be measured only to a minor extent. In accordance with the invention, this object is achieved by a fluid sensor which comprises: a flow sensor means which is arranged in a fluid flow; a differential pressure sensor which produces a differential pressure signal; a pump means which is constructed such that it is capable of superimposing an altemating flow on the flow to be measured, and which is characterized in that the flow sensor means is an essentially linear flow resistor; that the differential pressure sensor, the pump means, and the flow resistor are arranged parallel to each other, i.e., parallel with regard to the fluid flow; and that the pump means comprise at least one diaphragm and an actuating member for actively deflecting said at least one diaphragm. In a preferred embodiment of the invention, the differential pressure sensor is provided with a diaphragm having an opening which extends through said diaphragm at right angles to the main surfaces thereof, said diaphragm being provided with elements for measuring the diaphragm deflection. In another preferred embodiment of the invention, the pump means is provided with an additional diaphragm and an associated actuating member for actively deflecting said diaphragm, and the diaphragms of the pump means are provided with elements for measuring the diaphragm deflection, respectively. In a further preferred embodiment of the invention, the elements for measuring the diaphragm deflection include one strain gauge or several strain gauges. In a further preferred embodiment of the invention, the actuating members for deflecting the diaphragms of the pump means are controlled with an 180° phase shift for achieving thus a movement of these diaphragms in opposite directions. In a further preferred embodiment of the invention, a first chamber is defined by the diaphragm of the pressure sensor and by a diaphragm of the pump means and a sensor housing; a second chamber is defined by the diaphragm of the pressure sensor and by the additional diaphragm of the pump means and the sensor housing; the first and the second chambers are interconnected via the flow resistor; a means is provided for supplying the fluid to be measured to the first chamber, and a means is provided for discharging the fluid to be measured from the second chamber. In a further preferred embodiment of the invention, the sensor housing is provided with a flow passage for defining the flow resistor, said flow passage extending from the first chamber along one main side of the diaphragm of the differential pressure sensor through the opening of said diaphragm and along the other main side of said diaphragm to the second chamber. In a further preferred embodiment the sensor housing has a structural design of such a nature that the free diaphragm surface of the diaphragms of the pump means is larger than that of the diaphragm of the differential pressure sensor. In a further preferred embodiment, the supply means includes an inlet capillary leading through a cover portion and the sensor housing into the first chamber; and the discharge means includes an outlet capillary leading through the sensor housing and the cover portion and then out of the second chamber. In a further preferred embodiment, the first and second actuating members of the pump means have the following features: a first rear wall consisting of a thermally conductive material, said first rear wall being arranged on one lateral surface of the sensor housing and having a plurality of ribs; a second rear wall consisting of a thermally conductive material, said second rear wall being arranged on the other lateral surface of the sensor housing and having a plurality of ribs; a third chamber defined by the first diaphragm of the pump means, the first rear wall and the sensor housing, said third chamber containing a liquid; a fourth chamber defined by the additional diaphragm of the pump means, the second rear wall and the sensor housing, said fourth chamber containing a liquid; and first and second Peltier elements for cyclically heating and cooling down the liquids contained in the third and fourth chambers. In a further preferred embodiment, the first and second actuating members of the pump means each include a piezoelectric actuating element. In a further preferred embodiment, the first and second actuating members of the pump means each include a piston which is moved by magnetic fields. In another preferred embodiment, the first and second actuating members of the pump means are provided with chambers which are acted upon by a pressure. In accordance with another aspect of the invention, the fluid sensor comprises a volumetric flow rate determination means, thus enabling the measurement of the fluid flow rate, with the measurement being independent of or only slightly influenced by the properties of the fluid to be measured, such as the viscosity thereof. According to a further aspect of the invention, the fluid sensor comprises a signal evaluation means determining a value representative of the viscosity of the fluid, thus enabling the measurement of the viscosity of the fluid. BRIEF DESCRIPTION OF THE DRAWINGS In the following, preferred embodiments of the present invention will be explained in detail with reference to the drawings enclosed, in which: Fig. 1 shows a schematic representation of the pressure sensor according to the present invention; Fig. 2 shows a graph which, on the basis of some examples, shows the dependence of the sensor signal on the viscosity of the fluid; Fig. 3 shows a graph which corresponds to the graph of Fig. 2 and which shows how a small alternating flow introduced is transformed into a pressure signal which is proportional to the fluid properties; Fig. 4 shows a sectional view of an embodiment of a fluid sensor according to the present invention; Fig. 5 shows a block diagram of a device which uses the fluid sensor according to the present invention for determining the viscosity of a fluid and the volumetric flow rate of a fluid, respectively; and Fig. 6 shows a fluid sensor according to the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Prior to discussing a preferred embodiment of the fluid sensor according to the present invention, the measuring principle of the present invention will be explained in detail with reference to Fig. 1 to 3. Fig. 1 shows a fluid sensor according to the present invention designated generally by reference numeral 100. This fluid sensor 100 includes an essentially linear flow resistor 110 which is arranged in a fluid flow to be measured. A differential pressure sensor 120, which produces a differential pressure signal, is connected in parallel to the flow resistor 110. A pump 130 superimposing an alternating flow on the flow to be measured is arranged in parallel to the flow resistor 110 and the differential pressure sensor 120. This alternating flow must be known so as to provide the possibility of a regular self-calibration of the sensor. Fig. 2 shows the dependence of a sensor signal on the fluid viscosity on the basis of some examples, such as e.g. isopropanol, water, methanol or hexane. In practical operation, also arbitrary mixtures of fluids are used so that any ascending gradient between the characteristic curves shown is possible. Viscosities which are higher or lower than those shown in Fig. 2 are possible as well. Hence, the user will not be able to predict the viscosity of the fluid used. The pressure difference Δp across the fluid resistor is calculated in accordance with the following equation: Δ p = V x k wherein k stands for a proportionality factor and V stands for the flow to be measured. From the above it is evident that, in practice, the proportionality factor k mentioned in equation 1 is unknown. Fig. 3 shows the characteristic lines shown in Fig. 2, when an alternating flow 300 is superimposed on the flow V to be measured. In view of the fact that the magnitude of the superimposed alternating flow 300 is known, the respective proportionality factor k for the fluid in question can be determined on the basis of the alternating flow representation 310a - 310d in the differential pressure signal Δp. This is done without any previous knowledge of the fluid properties of the fluid. In order to guarantee a precise measurement, the fluid sensor and the system must be decoupled to a sufficient degree. This decoupling can be effected e.g. by flow resistors located ahead of and behind the fluid sensor in the direction of flow. Furthermore, the influence of the system on the fluid sensor must be ascertained and compensated for. Insufficient coupling resistances in combination with large system capacities will otherwise have the effect that the superimposed alternating flow 300 will not flow through the flow resistor 110, but disappear fully or partly in the system. A preferred embodiment of the present invention will now be explained in detail with reference to Fig. 4. As has already been described with reference to Fig. 1, the fluid sensor 100 according to the present invention comprises a flow resistor 110, a differential pressure sensor 120 connected in parallel to said flow resistor 110, and a pump 130 connected in parallel thereto. Furthermore, the differential pressure sensor 120 comprises a first diaphragm 410 having a first opening 410a which extends through said diaphragm 410 at right angles to the main surfaces thereof. In addition, the first diaphragm 410 is provided with first elements which serve to measure the diaphragm deflection. The pump 130 comprises second and third diaphragms 412, 414 and first and second actuating members. Said first and second actuating members are arranged on one side of the diaphragms 412, 414 and are used for actively deflecting the second and third diaphragms 412, 414. Just as the first diaphragm 410, the second and third diaphragms 412, 414 comprise second and third elements which serve to measure the deflection of the diaphragms 412, 414. To the person skilled in the art it will be obvious that these first, second and third elements for measuring the diaphragm deflection may be provided with one or several strain gauges. For producing the alternating flow 300 which is to be superimposed, the first and second actuating members are controlled with an 180° phase shift, and this has the effect that the second and third diaphragms 412, 414 will move in opposite directions. As can be seen in Fig. 4, the fluid sensor 100 comprises a first chamber 416 defined by the first and second diaphragms 410, 412 and a sensor housing 418. A second chamber 420 is defined by the first and third diaphragms 410, 414 and the sensor housing 418. The first and second chambers 416, 420 are interconnected via the flow resistor. The fluid sensor 100 is additionally provided with a means 422 for supplying the fluid to be measured to the first chamber 416 as well as with a means 424 for discharging the fluid to be measured from the second chamber 420. In the embodiment of the fluid sensor 100 shown in Fig. 4, the sensor housing 418 is provided with a flow passage 426 for defining the flow resistor 110. This flow passage 426 extends from the first chamber 416 along one main side of the first diaphragm 410 through the opening 410a of the first diaphragm 410 and along the other main side of the first diaphragm 410 to the second chamber 420. The sensor housing 418 has a structural design of such a nature that the free diaphragm surface of the second and third diaphragms 412, 414 is larger than that of the first diaphragm 410. In the preferred embodiment of the fluid sensor 100 according to the present invention, the supply means 422 includes an inlet capillary leading through a cover portion 428 of the sensor housing 418 into the first chamber 416. Furthermore, the discharge means 424 includes an outlet capillary leading through the sensor housing 418 and the cover portion 428 and then out of the second chamber 420. In the preferred embodiment of the fluid sensor according to the present invention, the first actuating member of the pump 130 has a first rear wall 430 consisting of a thermally conductive material, said first rear wall 430 being arranged on one lateral surface of the sensor housing 418 and having a plurality of ribs 430a. Furthermore, the second actuating member includes a second rear wall 432 consisting of a thermally conductive material, said second rear wall 432 being arranged on the other lateral surface of the sensor housing 418 and having also a plurality of ribs 432a. The second diaphragm 412, the first rear wall 430 and the sensor housing 418 define a third chamber 434. Furthermore, a fourth chamber is defined by the third diaphragm 414, the second rear wall 432 and the sensor housing 418. The third and fourth chambers 434, 436 each contain a liquid. In order to achieve the desired pumping effect of the pump 130, the first and second actuating members each include a Peltier element 438, 440 for cyclically heating and cooling down the liquid contained in the third and fourth chambers 434, 436. To those skilled in the art it will be obvious that the first and second actuating members of the pump 130 may have different structural designs. The first and second actuating members may, for example, each comprise a piezoelectric actuating element or a piston which is moved by magnetic fields. Furthermore, the first and second actuating members may be defined by chambers which are acted upon by a pressure. Whereas the structural design and the mode of operation of the fluid sensor 100 according to the present invention have been described hereinbefore with reference to Fig. 4, various devices making use of the sensor according to the present invention will be described hereinbelow with reference to Fig. 5. Fig. 5 shows a block diagram with the fluid sensor 100, a circuit for determining the fluid properties 510 and a circuit for determining the volumetric flow rate 512. The fluid sensor 100 is arranged in a fluid flow 514, the fluid entering the fluid sensor 100 on one side 514a thereof and leaving it on the other side 514b thereof. The output signal of the sensor 100 is supplied to the circuit 510 via a signal line 516, and via a node K1 and a second signal line 518 it is supplied to a first input 512a of the circuit 512. In the circuit 510 for determining the fluid properties, a value representative of the viscosity is determined by a signal evaluation means on the basis of the alternating differential pressure signal which is generated by the differential pressure sensor 120 and which is essentially proportional to the alternating flow 300. Through the output 510a of the circuit 510, this value is supplied via the third signal line 520 to a second input 512b of the circuit 512, whereupon it is tapped via the node K2 as output signal FV. The circuit 512 for determining the volumetric flow rate derives a quantity representative of a fluid property from the alternating differential pressure signal which is generated by the differential pressure sensor 120 and which is essentially proportional to the alternating flow, and it derives the volumetric flow rate V from the steady component of the differential pressure signal generated by the differential pressure sensor 120 as well as from the quantity FV which is representative of the fluid property. The value indicating the volumetric flow rate V is outputted via the output 512c of the circuit 512.

Claims: A fluid sensor comprising: a flow sensor means which is arranged in a fluid flow; a differential pressure sensor which produces a differential pressure signal ; a pump means which is constructed such that it is capable of superimposing an alternating flow on the flow to be measured, characterized in that the flow sensor means is an essentially linear flow resistor (110); the differential pressure sensor (120), the pump means (130), and the flow resistor (110) are arranged parallel to each other, i.e. parallel with regard to the fluid flow; and the pump means (130) comprise at least one diaphragm (412) and an actuating member for actively deflecting said at least one diaphragm. A fluid sensor as in claim 1, wherein the pump means (130) comprise an additional diaphragm (414) and an actuating member for actively deflecting the additional diaphragm, and wherein each of the diaphragms (412, 414) is provided with an element for measuring the diaphragm deflection. A fluid sensor as in claim 1 or 2, wherein the differential pressure sensor (120) is provided with a diaphragm (410) having an opening (410a) which extends through said diaphragm (410) at right angles to the main surfaces thereof, said diaphragm (410) being provided with elements for measuring the diaphragm deflection. A fluid sensor as in claim 2 or 3, wherein the elements for measuring the diaphragm deflection include one or several strain gauges. A fluid sensor as in any of the claims 2 to 4 , wherein the actuating members for deflecting the diaphragms (412,414) of the pump means are controlled with a 180 degrees phase shift for achieving thus a movement of these diaphragms (412,414) in opposite directions. A fluid sensor as in any of the preceding claims, wherein a first chamber (416) is defined by the diaphragm (410) of the pressure sensor and by a diaphragm (412) of the pump means and a sensor housing (418); a second chamber (420) is defined by the diaphragm (410) of the pressure sensor and by the additional diaphragm (414) of the pump means and the sensor housing (418); the first and second chambers (416,420) are interconnected via the flow resistor (110); a means (422) is provided for supplying the fluid to be measured to the first chamber (416); and a means (424) is provided for discharging the fluid to be measured from the second chamber (420). A fluid sensor as in claim 6, wherein the sensor housing (418) is provided with a flow passage (426) for defining the flow resistor (110), said flow passage (426) extending from the first chamber (416) along one main side of the diaphragm (410) of the differential pressure sensor through the opening (410a) of said diaphragm (410) and along the other main side of said diaphragm (410) to the second chamber (420). A fluid sensor as in any of the preceding claims, further comprising a signal evaluation means (510) determining a value representative of the viscosity (FV) on the basis of the alternating differential pressure signal which is generated by the differential pressure sensor (120) and which is essentially proportional to the alternating flow (300). A fluid sensor as in any of the claims 1 to 7, further comprising a volumetric flow rate determination means (512) which derives a quantity (FV) representative of a fluid property from the alternating differential pressure signal which is generated by the differential pressure sensor (120) and which is essentially proportional to the alternating flow (300), and which derives the volumetric flow rate (V) from the steady component of the differential pressure signal generated by the differential pressure sensor (120) as well as from the quantity (FV) representative of the fluid property.

Inventors: HAERTL HANS-GEORG, HAERTL, HANS-GEORG, Härtl, Hans-Georg
Applicants: HEWLETT PACKARD GMBH, HEWLETT-PACKARD GMBH
Application year: 1994
Main classification: G01F1
IPC codes: G01N11, G01F1, G01F15